1. Field of the Invention
The present invention relates to a process for preparing a polymer compound for a resist and, more particularly, to a process for preparing a polymer compound for a resist which is useful as a photosensitive resin such as a resist, or as a precursor used to obtain the photosensitive resin.
2. Description of the Related Art
A homopolymer or copolymer of 4-hydroxystyrene is known as a photosensitive resin such as a resist, or a precursor used to obtain the photosensitive resin.
As the process for preparing such a polymer, for example, there has been known a process for preparing a vinylphenol polymer, which comprises reacting a polymer of 4-acetoxystyrene dissolved in methanol with a quaternary ammonium base at a temperature within a range from about 40xc2x0 C. to about 80xc2x0 C. over sufficient time to cause hydrolysis of an acetoxy group to form a phenol group, and heating to a temperature within a range from about 50xc2x0 C. to about 150xc2x0 C., thereby to distill off the methyl acetate produced during the reaction and a decomposition product of a quaternary ammonium hydroxide, as disclosed in Japanese Examined Patent Application, Second Publication No. Hei 7-96569 B.
Control of the composition ratio and the molecular weight of the polymer, and reducing the amount of solution residue in alkali solutions have hitherto been carried out to improve the performance such as the heat resistance, sensitivity and resolution of the resist. Particularly, modification of the polymer with a specific compound has hitherto been conducted to improve the resolution.
However, control of the polymer composition ratio and hydrophilization of the polymer by modifying the same gave rise to problems such as film loss of the resist and line width reduction of the resist. Also reduction of the molecular weight of the polymer improves the solubility in alkali solutions, but causes problems such as deterioration of the dry etching resistance of the polymer.
According to the technique disclosed in the above Japanese Examined Patent Application, Second Publication No. Hei 7-96569 B, it is possible to remove residual alkali metals which give rise to problems, by utilization of a 4-hydroxystyrene polymer. However, it was difficult to improve the performance such as the heat resistance, sensitivity and resolution of the resist without causing film loss of the resist, line width reduction of the resist and deterioration of the dry etching resistance of the polymer.
An object of the present invention is to provide a process for preparing a polymer compound for a resist which can improve the performance, such as the heat resistance, sensitivity and resolution of the resist, without causing film loss of the resist, line width reduction of the resist or deterioration of the dry etching resistance of the polymer.
The present inventors have carried out intensive studies to achieve the object described above and found that the problems described above can be solved by adding an alkali catalyst to a solution of a polymer prepared from as at least one of the constituent monomers, a 4-acetoxystyrene-based monomer obtained by using a dimethyl-2,2xe2x80x2-azobiscarboxylate ester as a polymerization initiator, thereby to hydrolyze the polymer solution, and washing the resulting polymer compound with water. Thus, the present invention has been completed.
The present invention provides a process for preparing a polymer compound for a resist, which comprises dissolving a styrene-based monomer containing at least a 4-ace toxystyrene monomer, and a dimethyl-2,2xe2x80x2-azobiscarboxylate ester in an solvent, thereby to polymerize the styrene-based monomer; adding an alkali catalyst to the resulting polymer solution, thereby to hydrolyze the polymer; and washing the resulting polymer compound with water.
Such a constitution makes it possible to obtain a polymer compound for resist which can improve the performance, such as the heat resistance, sensitivity and resolution of the resist, without causing film loss of the resist, line width reduction of the resist or deterioration of the dry etching resistance of the polymer.
The styrene-based monomer used in the present invention contains at least a 4-acetdxystyrene monomer. The styrene-based monomer is preferably a compound represented by the following formula (1): 
R1 represents a hydrogen atom, a hydroxyl group, an alkyl group having 1 to 6 carbon atoms, an alkoxy group having 1 to 6 carbon atoms, or an acyloxy group having 2 to 7 carbon atoms, and R2 represents a hydrogen atom or a methyl group. The alkyl group having 1 to 6 carbon atoms, the alkoxy group having 1 to 6 carbon atoms and the acyloxy group having 2 to 7 carbon atoms may be straight or branched. R1 is more preferably a hydroxyl group, an alkoxy group having 3 to 5 carbon atoms, or an acyloxy group having 2 to 4. carbon atoms, and particularly preferably a hydroxyl group, a t-butoxy group, or an acetoxy group. In the present invention, a 4-acetoxystyrene monomer and, if necessary, one or more kinds of the styrene-based monomers described above can be used.
A dimethyl-2,2xe2x80x2-azobiscarboxylate ester is used as a polymerization initiator for polymerizing the styrene-based monomer. A carboxylic acid residue, which constitutes the di methyl-2,2xe2x80x2-azobiscarboxylate ester, may be straight or branched, and preferably has 2 to 8 carbon atoms, and more preferably 2 to 5 carbon atoms. A 2-methylpropoxy group is particularly preferred. The use of the dimethyl-2,2xe2x80x2-azobiscarboxylate ester as the polymerization initiator makes it possible to obtain a polymer compound for resist which can improve the performance, such as the heat resistance, sensitivity and resolution of the resist without causing film loss of the resist, line width reduction of the resist or deterioration of the dry etching resistance of the polymer. When using 2,2xe2x80x2-azobisisobutyronitrile as the polymerization initiator, the excellent effects described above cannot be obtained to a sufficient extent.
According to the process of the present invention, first, a styrene-based monomer containing at least a 4-acetoxystyrene monomer, and a dimethyl-2,2xe2x80x2-azobiscarboxylate ester are dissolved in an solvent, thereby to polymerize the styrene-based monomer to prepare a polymer containing the styrene-based monomer as a constituent monomer.
The solvent, which dissolves the styrene-based monomer and the dimethyl-2,2xe2x80x2-azobiscarboxylate ester, thereby to polymerize the styrene-based monomer, is not specifically limited as far as it can dissolve them, and a general solvent can be used, but the solvent preferably contains no methanol. Specific examples thereof include ethylene glycol monoethyl ether and propylene glycol monomethyl ether acetate.
The polymerization can be conducted, for example, by a solution polymerization, suspension polymerization, emulsion polymerization or block polymerization process using a publicly known free radical catalyst such as a peroxide or an azo compound.
The mixing ratio (molar ratio) of the styrene-based monomer to the dimethyl-2,2xe2x80x2-azobiscarboxylate ester is preferably within a range from 100:1 to 100:30, and particularly preferably from 100:2 to 100:20. With respect to the reaction conditions, the reaction temperature is preferably within a range from 1 to 150xc2x0 C., and particularly preferably from room temperature (25xc2x0 C.) to 100xc2x0 C., while the reaction time is preferably within a range from 1 to 48 hours, and particularly preferably from 3 to 24 hours. The atmosphere in the reaction vessel is preferably replaced by nitrogen to prevent the deactivation of radicals.
By appropriately selecting these reaction conditions, a polymer having a molecular weight suited for use can be prepared.
Then, the resulting polymer is hydrolyzed by adding an alkali catalyst to prepare a hydroxystyrene polymer. The alkali catalyst is preferably a quaternary amine hydroxide. The quaternary amine hydroxide is represented by the following formula. 
In the formula, each R is an organic group having 1 to 9 carbon atoms. R may be aliphatic or aromatic and may be the same or different. The organic group also includes hydroxyethyl. Examples of the organic group include methyl, ethyl, isopropyl, butylbenzyl, methylbenzyl, and dimethylbenzyl. Examples of useful quaternary ammonium hydroxide include tetramethylammonium hydroxide, ethyltrimethylammonium hydroxide, diethyldimethylammonium hydroxide, triethylmethylammonium hydroxide, tetraethylammonium hydroxide, benzyltrimethylammonium hydroxide, and choline. Aparticularly preferred quaternary ammonium hydroxide is tetramethylammonium hydroxide.
The amount of the alkali catalyst to be added is preferably within a range from 1 to 20% by weight, and particularly preferably from 1 to 5% by weight, based on the amount of the styrene-based monomer. With respect to the hydrolysis conditions, the hydrolysis temperature is preferably within a range from 20xc2x0 C. to 100xc2x0 C., and particularly preferably from 20xc2x0 C. to 80xc2x0 C., and the hydrolysis time is preferably within a range from 1 to 10hours, and particularly preferably from 3 to 8 hours.
The resulting hydroxystyrene polymer-containing solution is then washed with water. Washing with water makes it possible to obtain a polymer compound for resist which can improve the performance, such as the heat resistance, sensitivity and resolution of the resist, without causing film loss of the resist, line width reduction of the resist or deterioration of the dry etching resistance of the polymer. Because they are not stable to heating, quaternary ammonium hydroxides have the merit that they decompose into a low-boiling decomposition product after the hydrolysis reaction, resulting in easy removal from the reaction solution. However, it is difficult to completely remove quaternary ammonium hydroxides only by decomposition with heating. Therefore, when using decomposition with heating in place of washing with water in order to remove the quaternary ammonium hydrides, the excellent effects described above cannot be obtained to a sufficient extent. Washing can be conducted by adding an arbitrary amount of water an arbitrary number of times. The temperature for the addition is not specifically limited. Water may be mixed with methanol in an appropriate ratio.
A polymer compound for a resist can be obtained by purifying and drying the solution using a conventional process after washing with water.
Similarly to the homopolymerization of styrene, 4-acetoxystyrene can be easily homopolymerized, or copolymerized with styrene or another monomer capable of copolymerizing with styrene.
In the present invention, a compound other than the styrene-based monomer can be copolymerized together with the styrene-based monomer. Examples of such a compound include vinyltoluene, a-ethylstyrene, a diene monomer such as butadiene; acrylate such as methyl acrylate, methyl methacrylate, ethyl acrylate, butyl acrylate, butyl methacrylate, or 2-ethylhexyl acrylate; and polymerizable acid such as methacrylate ester monomer, acrylic acid, methacrylic acid, maleic anhydride, maleic acid, or fumaric acid. The compound is preferably a compound represented by the following formula (2): 
wherein R3 represents a hydrogen atom or a methyl group, and R4 represents an alkyl group having 1 to 6 carbon atoms, or any one of the following formulas (3) to (7): 
The effect of improving the resolution is exerted by polymerizing the compound represented by the formula (2) as a monomer. The mixing ratio of the styrene-based monomer to the compound represented by the formula (2) is not specifically limited.
The polymer compound for a resist prepared by the process of the present invention is useful as a resin for a resist used in the formation of resist patterns. The resist pattern can be formed, for example, by preparing a resin solution containing the polymer compound for a resist, a photo acid generator and a solvent (for example, ethyl lactate), applying the resist solution onto a substrate to form a resist film, exposing the resist film to light and developing the exposed resist film.